Hydrolysis of the organic acid esters of cellulose



Sept. 10, 1935. c J. MALM AL 2,013,830

HYDROLYSIS OF THE ORGANIC ACID ESTERS OF CELLULOSE Filed Jan. 11, 1933- 2 Sheets-Sheet 1 5w Fi l 275 J E 225 AVERAGE FIRST STAGE VISCOSITY 1200 o 200 CENTIPOISES. 7 POUNDS FIRST STAGE ACETATE m SOLUTION 45.5 POUNDS. 2 TOTAL SOLVENTATFIRSTSTAGEJZ/LB. l- POUNDS H2O PEPPOUND OF BONE DRY I25 COTTON AT FIRSTSMGE 2w POUNDS. I00 TEMPERATURE OFHYDROLYSIS I00E I POUNDS PEI? POUND BONE DRY COTTON.

L58 PERLB. (XJTTON.

.I67LB8. H 80, PER LB.

.wLBsJ s PEPLacOTTOAA LBS. H 0AVAILABLEF0RACH LB. BONE DRY COTTOVAT. STAGE.

Z0 40 60 60 I00 I I I M220 2 D 2wZ605005205405b0 TIME OF HYDPOLVSIS.

Sept. 1935. C J MALM ET AL 2,013,830

HYDROLYSIS OF THE ORGANIC ACID ESTERS OF CELLULOSE Filed Jan. 11, 1933 2 Sheets-Sheet 2 VISCOSITY or4:/ ACE TONE SOLUTION m secomns. 8 g 8 93" 80 10/20/40 160180 200220240 2b0'250500520540560 7 TIME OF HYDROLYSIS EXPERIMENT /1 80, %H 0 A 1.25 25 B .25 25 C L25 5 D gwovntom .rzwummmmwfle im Patented Sept. 1%, 1935 HYDROLYSIS OF THE ORGANIC ACID ESTERS F CELLULOSE Carl J. Malm and Charles L. Fletcher, Rochester,

assignors to Eastman Kodak Company,

Rochester, N. Y., a corporation of New York Application January 11, 1933, Serial No. 651,138

6 Claims.

The present invention relates to the hydrolysis of the organic acid esters of cellulose, such as cellulose acetate, cellulose acetate-propionate and the like, in a bath containing a relatively high percentage of water.

Up to the present time the fully esterified cellulose acetates, which are acetone-insoluble, have not found much use commercially, however, in practice they have been hydrolyzed to acetonesoluoility which material has become of great commercial importance. The cellulose esters of the fatty acids higher than acetic include a good many compounds which are acetone-soluble, however, it is sometimes desirable to hydrolyze acetone-soluble esters of cellulose to alter their solubilities or improve their characteristics as for example in the process of hydrolyzing mixed esters of cellulose which are acetone-soluble which was disclosed and claimed in U. S. application Serial No. 551,546 of Malm and Fletcher, filed July 17, 1931. The method usually employed for the hydrolysis of cellulose acetate which has been produced by a non-fibrous formula, is to add to the completed reaction mixture containing fully esterified cellulose acetate an amount of water but little in excess of that necessary to kill the anhydride present in the bath. This addition of water in practice is carried out by adding the calculated amount of aqueous acetic acid to the reaction mixture. Usually the amount of water permitted'in the bath does not exceed In no case which we have found, has the prior art ever employed much more than 10% water. In those processes it was necessary, if degradation was to be avoided, to maintain the temperature low and to restrict the proportion of mineral acid added.

As a consequence the rate of hydrolysis was relatively small and the time necessary for this step resulted in the tieing up of equipment for relatively long periods. It may be observed, for instance, from the data in U. S. Patent No. 1,782,796 of Vles that hydrolysis maybe speeded up when a higher temperature is employed, however a considerable and undesired decomposition of the cellulose unit results.

One object of our invention is to provide a process for hydrolyzing an organic acid ester of cellulose in a relatively short period without degradation of the cellulose unit. Another object of our invention is to prepare an organic acid ester of cellulose, such as a cellulose acetate, in a shorter time and with higher viscosity, flexibility, tensile strength, etc, than an ordinary hydrolyzed cellulose acetate. A further object of our invention is to provide a hydrolysis process for cellulose esters in which a relatively large amount of water may be employed in the hydrolysis bath.

The rate at which a hydrolysis process proceeds is dependent chiefly upon the relative amount of sulfuric acid or other equivalent catalyst which is present in the hydrolyzing bath compared to the amount of cellulose which was employed initially as the starting material for the esterification. accompanying drawings which is based upon the data collected from several hydrolysis series in each of which all other factors but the amount of catalyst were kept constant.

This is illustrated by Figure I of the 10 An esterification dope containing a cellulose acetate having a tetra-chlorethane viscosity of about 1286 centipoises was employed. This esterification dope contained 43.5 lbs. of acetate and 331 lbs. of acetic acid as solvent. Water was added in the proportion of 2.05 lbs. of water per pound of bone dry cellulose employed as the starting material of the acetylation. As lbs. of cellulose was initially employed the amount of water added was 51.25 lbs. The temperature in each case was maintained at 100 F. thruout the hydrolysis. From this data the equation was calculated and the remainder of the points to complete the curve were figured from this equation, in which y is the time of hydrolysis, in hours, required to produce a cellulose acetate having acetyl and a is the number of pounds of 95% sulfuric acid in the hydrolysis bath per pound of dry cotton. When a sufficient amount of sulfuric acid is added to appreciably reduce the time of hydrolysis in the ordinary hydrolysis bath employed by those skilled in the art it has long been known that a certain amount of degradation, particularly after partial hydrolysis of the ester has occurred, was an inavoidable evil of the process.

We have now found that the degrading action of a fast hydrolysis bath (that is containing sufilcient sulfuric acid for a relatively quick hydrolysis) may be inhibited or buffered by the presence of a relatively large amount of water in the hydrolysis bath without the necessity of employing buffering salts and the like which slow down the reaction and otherwise complicate it.

For example enough water may be added so that the bath has only an average fatty acid concentration of about 70% or even less, depending upon the conditions'and materials employed, up to 88%.

We have found that this increase of the water content of the hydrolysis bath also aids in increasing the rate of hydrolysis, thus making the time necessary to reach a certain acyl content even less than would be the case if the proportion of sulfuric acid only were increased. This increase of rate of hydrolysis with the increased water content is illustrated in Figure II in which it is to be noted that the rate of increase of the time of hydrolysis in each instance (i. e. slope of line) with the addition of water is a straight line function independent of the amount of sulfuric acid present; hence the lines representing the different contents of sulfuric acid are parallel showing that for any given temperature and percentage of sulfuric acid, based on the cellulose, the amount of water in the bath alone controls the speed of hydrolysis.

The range of water content for a hydrolysis bath covered by our invention includes not only baths which are solvent of the fully esterified ester but baths which will precipitate it out at room temperature. In the case of the first type the water may be added entirely at the beginning of the hydrolysis. In practice this is done by adding the water in the form of dilute acetic acid preferably of suificient concentration (such as about 50%) so that it may be added without great care being necessary to avoid precipitation.

Inthec'ase of the use of a water content which if added initially will precipitate the fully esterifie'd ester it is necessary either that the water be added while the whole system is kept at elevated temperature or that it be added in steps.

. Advantage may be taken of the facts that the fully esterified ester is not as easily degraded as one which has been partially hydrolyzed and that the hydrolyzed ester is soluble in acetic acid of greater dilution than the fully esterified ester that the water (in the form of dilute acetic acid) may be added either in steps or continuhydrolysis where breakdown of the cellulose unit is likely to occur. If it is desired to add the water at an elevated temperature, the reaction mixture should be heated to a temperature of 120-150 F. The temperature of the dilute acetic acid (of quite low concentration) should also be raised above 120 F. before it is added to the reaction mixture. After this addition to form a bath containing in the vicinity of 20-25% water it should be maintained above 120 F. until some hydrolysis has occurred after which the temperature may be lowered if desired, provided the ester present remains in solution in the bath.

The temperature of the hydrolysis bath however need not be elevated if the water is added at intervals or continuously over a portion of or during the whole of the time consumed in the hydrolysis. As the ester becomes more and more compatible with water the percentage of water in the hydrolysis bath may be correspondingly increased.

Although the continuous addition of water so that the bath may at all times contain the maximum amount of water which may be present and not precipitate the ester, is preferred due to the maximum buffering action obtained thereby, the addition of water to the bath at frequent intervals has been found to be quite satisfactory in practice providing of course the maximum amountof water which may be added Without precipitation of the ester from the bath is employed upon each addition.

As illustrative of the results to be obtained by our invention, but by which we are in no way to be limited, the following is exemplary:

A reaction mixture containing cellulose acetate was prepared by reacting upon 25 lbs. of dry cotton linters with '71 lbs. of acetate anhydride 160 lbs. of glacial acetic acid and 110 c. c. of sulfuric acid in the usual way until the dope was free of grain. The temperature was raised to F. and a final mixture consisting of acetic acid, sulfuric acid and water, in predetermined proportions, depending on the conditions desired, was added.

In the case where a hydrolysis bath of a concentration that will retain the ester in solution is desired the final may be 50% aqueous acetic" acid containing the desired amount of sulfuric acid which, if at room temperature, should be added slowly over a period of about 15 minutes by pouring it directly into the dope and allowing it to dissolve therein. For example if in the above esterification mixture it is desired to dilute the acetic acid to an 85% concentration, by a simple calculation it will be found that about 89 lbs. of 50% aqueous acetic acid will give the desired concentration.

In the case where a percentage of water is desired in the hydrolysis bath which would render that bath non-solvent of the cellulose ester therein, for example, a water concentration of approximately 2-5%, a final of 20% aqueous acetic acid is preferred to keep down bulkiness. The procedure followed was (1) addition at 100 F. of a mixture of all the mineral acid catalyst and enough of the 20% aqueous acetic acid to be slightly in excess of that necessary to convert the excess acetic anhydride to acid, such addition requiring about 15 minutes, (2) slowly introducing the remainder of the aqueous acetic acid into the dope, allowing it to trickle onto the surface thereof thru a small jet. The rate of addition of water was dependent upon the sulfuric acid present andv temperature of the water, it being so regulated that 'all of the final was added by the time the ester became just soluble in acetone. If for example a hydrolysis bath is desired in which the amount of water added-if added initiallywould dilute the acetic acid to- 75% concentration, it may be calculated that about '77 lbs; of 20% aqueous acetic acid will give the desired amount of water.

After the addition of the. final by any of the methods described the dope was held at a temperature of 100 F., the time over which it may be allowed to stand depending on'the acetyl content desired and the amounts of sulfuric acid used.

The quantity of sulfuric acid employed will depend on the speed desired, and may be ascertained from the curve of Figure I, under the conditions there represented. It may be seen,

however, from that curve that amounts of sulhydrolysis will be increased so that the addition of less sulfuric acid than .2 lb. per pound of dry cotton will be satisfactory to attain the same speed of hydrolysis.

The degradation effected upon cellulose acetate by various hydrolysis baths may be compared by hydrolysis of samples of a certain cellulose acetate in each of the various baths herein described and comparing the minimum acetone viscosities (a measure of degradation) of the materials prepared by each bath. The term minimum acetone viscosity is that point during the hydrolysis of cellulose acetate at which the acetate, when dissolved in acetone at a given concentration, has the minimum viscosity.

Figure III illustrates a comparison between 4 hydrolysis baths as to their degrading effects. Curve A shows the acetone viscosity of an acetate treated with a hydrolyzing bath having a content of 1%;% of 95% sulfuric acid (based on the cellulose) and of water (based on the bath). It will be observed from the curve that the hydrolysis took place quickly, as a consequence of the large percentage of sulfuric acid which was present in the bath. It will also be observed that the minimum viscosity reached was approximately double that of the products forming the basis of curves C and D. Thus degradation is avoided in the case of curve A due to the large water content of the hydrolyzing bath even through five times as much sulfuric acid was present as in the case of curve D (but which em ployed low total water). Curve B is based on a hydrolysis bath having a content of 25% water and 4% of sulfuric acid (95%). The hydrolysis proceeded at a slower rate than before due to the diminished amount of sulfuric acid present. However, because of the relatively large amount of water present in the bath the minimum viscosity reached was comparatively high and degradation avoided. In the hydrolysis upon which curves C and D are based the hydrolysis baths each contained only 5% of water, the C and D hydrolysis baths containing respectively l /4% and A% of 95% sulfuric acid. In these curves not only is the minimum viscosity lower, showing that degradation of the cellulose molecule has occurred, but the rate of hydrolysis is less in the respective examples where a large quantity of water was employed. Furthermore a comparison of curve A with curve B shows that so long as a large amount of water is present in the bath, the sulfuric acid may be increased as much as five times without giving degradation.

It is to be understood that variation of the temperature employed in the hydrolysis will change the rate of hydrolysis. For instance in Figure I that step was carried out at 100 F. however, if it were performed at room temperature (70 F.) the rate would of course be less. As the water in the present invention acts as a buifer to degradation, elevated temperatures may be safely employed depending on the proportion of water present in the bath. Obviously the greater the percentage of water present in the bath, the higher the temperature may be allowed to go with avoidance of serious degrading action. It is also apparent that with an increase in temperature the rate of hydrolysis also increases. In short by means of the relatively high water content in the hydrolysis bath in accordance with our invention, the various agencies which induce hydrolysis may be employed with much greater severity than heretofore without any greater degradation taking place than heretofore under milder conditions and at a much slower rate.

In the present invention the range of Water concentration permissible in the hydrolysis bath is from 12 to as much as or even more.

Where a maximum amount of buffering action against degradation is desirable such as where a large amount of sulfuric acid and/or a sharply elevated temperature is employed it will be desirable though probably not necessary that the hydrolysis bath employed contain a percentage of water in the upper parts of the water concentration range. In cases, however, where a small amount of buffering action is suii'icient, as under most conditions of operation, a percentage of water selected from the lower part of the water concentration range may be satisfactorily employed in the hydrolysis bath. In most cases we have found about 15-20% water to be the preferred range.

Although sulfuric acid is the preferred catalyst due to its superb power of hydrolyzing cellulose esters, other hydrolyzing catalysts such as sulfuric-phosphoric acids (1:3), phenol sulfonic acid or the acid sulfites, which are disclosed in U. S. Patent No. 1,878,953 of C. J. Malm, as well as other catalysts may be employed in quantities proportional to their catalytic activity. The use to which some catalysts may be put in practical operation will necessarily be limited if a high rate of hydrolysis is required due to the large amount of a weak catalyst which would be required in such a case.

The present invention is not confined to the hydrolysis of cellulose acetate only but is also applicable to the hydrolysis of other fatty acid esters of cellulose both simple and mixed, especially of the lower fatty acids, such as cellulose propionate, cellulose acetate-propionate, cellulose butyrate, cellulose acetate-butyrate and cellulose propionate-butyrate, the latter ester havingbeen disclosed and claimed in application Serial No. 520,149 of Clarke and Malm, filed March 4, 1931. The changes in solubilities which take place upon the hydrolysis of the mixed esters containing at least 15% of a fatty acid radical containing more than 2 carbon atoms are disclosed in our co-pending application Serial No. 551,546, filed July 1'7, 1931.

We have found that the time necessary. for hydrolyzing a cellulose acetate having a tetrachlorethane viscosity of 1200 centipoises at 100 F. to obtain a cellulose acetate having a predetermined acetyl content may be calculated approximately from the following formula.

Ac,Ac 7.375 T: 110(Rz2-05 In the above formula:

Acs=acetyl in per cent at beginning of hydrolysis Acx=acetyl desired in per cent T=time in hours to hydrolyze to ACx R1=pounds 95% sulfuric acid (including the acetylation catalyst) per lb. of bone dry cotton employed as starting material.

R2=pounds of total available water in the hydrolysis bath, per lb. of bone-dry cotton employed as starting material.

Obviously those skilled in the art can, if they desire, work out formula for other variations of the invention such as different temperatures, other catalysts, esters having a different viscosity and/ or containing other acyl groups than acetyl. For instance, we may combine step-wise addition of hydrolyzing agent with step-wise reduction of temperature 1. e. since at a higher temperature the hydrolyzing bath will tolerate more water, one may employ larger proportions of water in the initial stages of the hydrolysis, than otherwise, by employing higher initial temperatures and gradually reducing the temperature as the hydrolysis proceeds and the higher temperatures become less necessary to keep the ester in solution with the large amount of water employed.

Other compounds containing hydroxyl groups suggest themselves as suitable in place of water in the present hydrolysis process, such as ethyl or methyl alcohol, hydro-Ky acids such as lactic, etc. however, as water is the cheapest and at the same time is quite satisfactory it is to be preferred inthis connection. All of these various materials which will kill or convert the anhydride and which effect hydrolysis of cellulose esters will be referred to herein as hydrolyzing agents. By the term aggregate content as applied herein to the content of the water and other hydrolyzing agent in a hydrolysis bath is meant the total amount of the agent that has been supplied to the bath (some of which agent may have been inherent therein at the beginning of the hydrolysis) with reference to the sum of the liquids of the reaction mixture plus the added water or other agent.

Various other modifications of our invention which may be apparent to those skilled in the art are also to be considered as being within the scope of the appended claims.

We claim as our invention:

1. A process of hydrolyzing an organic acid ester of cellulose which comprises treating the ester with a bath which is given hydrolyzing properties by supplying water thereto, initially and over the major portion of the hydrolysis period, in an amount sufiicient to inhibit degradation of the ester but insufficient to render the bath non-solvent of the ester.

2. The process of hydrolyzing an organic acid ester of cellulose which comprises treating the ester in a hydrolyzing bath in which approximately the maximum amount of water compatible with the ester solution is supplied to the bath at short intervals over the major portion ofthe hydrolysis period.

3. A process of hydrolyzing an organic acid ester of cellulose which comprises treating the ester at an elevated temperature with a hydrolyzing bath containing the maximum amount of water compatible with the solution of the ester in the bath, and after a time lowering the temperature only to a point at which the amount of water present is still the maximum compatible amount which may be tolerated.

4. The process of hydrolyzing an organic acid ester of cellulose which comprises treating the ester in a hydrolyzing bath in which approximately the maximum amount of water compatible with the ester solution is supplied tothe bath, initially and at short intervals over the major portion of the hydrolysis period and in which the bath is heated initially to an elevated temperature to increase the tolerance for the water, this temperature being then allowed to gradually decrease.

5. The process of hydrolyzing an organic acid ester of cellulose with a hydrolyzing bath having an aggregate content of from 12% to of water, which comprises first treating the ester with a bath in which only a portion of the water is present until it becomes tolerant of more water, then adding further water to the bath and continuing the hydrolysis to the point desired.

6. The process of hydrolyzing an organic acid ester of cellulose which comprises treating the ester in a hydrolyzing bath in which approximately the maximum amount of water compatible with the ester solution is supplied to the bath in small increments over the major portion of the hydrolysis period.

CARL J. MALM. CHARLES L. FLETCHER. 

